WO2013124524A1

WO2013124524A1 - Method and apparatus for controlling aggregate interference in a cognitive radio network

Info

Publication number: WO2013124524A1
Application number: PCT/FI2012/050184
Authority: WO
Inventors: Fabiano CHAVES; Erika ALMEIDA; Robson DOMINGOS
Original assignee: Nokia Corporation
Priority date: 2012-02-23
Filing date: 2012-02-23
Publication date: 2013-08-29

Abstract

In accordance with an example embodiment of the present invention the application discloses a method and an apparatus for receiving a resource request (401 ); receiving information about locations of at least two white space devices; associating a location of a first white space device with a first maximum interference (402); associating at least one location of at least one second white space device with at least one second maximum interference (402); identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device (404); calculating a maximum permitted power level (405) for the first white space device at least partly based on determining an aggregate interference level caused by the two white space devices at the one or more focal points; and informing the maximum permitted power level to the first white space device (406).

Description

METHOD AND APPARATUS FOR CONTROLLING AGGREGATE INTERFERENCE IN A COGNITIVE RADIO NETWORK TECHNICAL FIELD

[0001] The present application relates generally to wireless communications in a cognitive radio network. More particularly the present application relates to selection of operating parameters for a plurality of TV white space devices such that the aggregate interference caused by the plurality of white space devices does not exceed regulatory levels.

BACKGROUND

[0002] Cognitive radio networks comprise devices, which are capable of obtaining knowledge of the surrounding radio environment and adapting their functionality accordingly. A cognitive radio device may also take into account user needs, user preferences, and/or other circumstances, and based on the knowledge available determine suitable means for communication. Cognitive radio systems generally employ dynamic spectrum allocation to achieve maximum flexibility.

[0003] Examples of cognitive radio networks are TV white space networks that may operate within traditional television frequencies. In general, a white space may be defined as a part of a frequency spectrum, which is available for secondary radio communication in certain geographical area. Cognitive white space devices (WSD) may be allowed to operate in such white spaces provided that the WSDs do not cause harmful interference to the users of the primary network. A white space may refer to a frequency band which is not allocated to any primary system, or, a white space may refer to a frequency band which is used by a primary system but in which the primary users are not harmfully interfered by the secondary users.

[0004] A primary network may be for example an incumbent radio service or any other system authorized to operate on certain geographical area and frequency band.

Examples of such primary, or incumbent, networks are the digital or analog terrestrial television systems such as DVB-T (Digital Video Broadcasting - Terrestrial), DVB-T2 (Digital Video Broadcasting - 2^nd generation Terrestrial), ATSC (standards specified the Advanced Television Systems Committee), DMB-T (Digital Terrestrial Multimedia Broadcast), or ISDB-T (Integrated Services Digital Broadcasting - Terrestrial). Further examples of incumbent networks include Program Making and Special Event (PMSE) systems such as radio microphones, Radio Astronomy Services (RAS) which may operate at a frequency band between 608 MHz and 614 MHz, Aeronautical Radio Navigation Services (ARNS) in a frequency band of 645-790 MHz, or mobile services below 470 MHz or above 790 MHz.

[0005] A wide range of devices may act as a white space device including for example any personal or portable devices such as mobile phones, media players, PDAs (personal digital assistants), or laptops; home or office devices such as computers, printers, televisions, or other home appliances; or private or public access points such as systems according to the IEEE 802.11 family of specifications (WLAN, Wireless Local Area Network or Wi-Fi) or the like.

[0006] In order to mitigate interference with incumbent services, white space devices of a cognitive secondary system may be able to identify vacant channels. Examples of such techniques are spectrum sensing and the use of geo-location databases. In case of spectrum sensing, the task of identifying primary transmission is performed at a secondary device, or alternatively at multiple secondary devices, through observation and analysis of the transmission environment. Example techniques for spectrum sensing include energy detection and cyclo stationary detection. In case of cognitive radio data bases, the identification of vacant channels may be performed at a database through the combination of knowledge about primary system deployment and the current location of the white space devices.

SUMMARY

[0007] Various aspects of examples of the invention are set out in the claims.

[0008] According to a first aspect of the present invention, a method comprises receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; receiving information about at least one location of at least one second white space device; associating the location of the first white space device with a first maximum interference; associating the at least one location of the at least one second white space device with at least one second maximum

interference; identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and informing the maximum permitted power level to the first white space device.

[0009] According to a second aspect of the present invention, an apparatus comprises at least one processor; at least one memory containing executable instructions, wherein the executable instructions, when processed by the processor, cause at least to: receive a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; receive information about at least one location of at least one second white space device; associate the location of the first white space device with a first maximum interference; associate the at least one location of the at least one second white space device with at least one second maximum interference;

identify one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; calculate a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and inform the maximum permitted power level to the first white space device.

[0010] According to a third aspect of the present invention a computer program product comprises a computer-readable medium bearing computer program code embodied therein for use with a computer, the computer program code comprising: a computer-readable medium bearing computer program code embodied therein for use with a computer, the computer program code comprising: code for receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; code for receiving information about at least one location of at least one second white space device; code for associating the location of the first white space device with a first maximum interference; code for associating the at least one location of the at least one second white space device with at least one second maximum interference; code for identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; code for calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and code for informing the maximum permitted power level to the first white space device.

[0011] According to a fourth aspect of the present invention, an apparatus comprises means for means for receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; means for receiving information about at least one location of at least one second white space device; means for associating the location of the first white space device with a first maximum interference; means for associating the at least one location of the at least one second white space device with at least one second maximum interference; means for identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; means for calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and means for informing the maximum permitted power level to the first white space device

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0013] FIGURE 1 illustrates an exemplary structure of a cognitive radio network in accordance with one or more embodiments of the invention

[0014] FIGURE 2 presents an exemplary structure of a geo-location database apparatus in accordance with one or more embodiments of the invention

[0015] FIGURE 3 presents an exemplary structure of a white space apparatus in accordance with one or more embodiments of the invention.

[0016] FIGURE 4 describes an exemplary algorithm for performing a method in accordance with one or more embodiments of the invention. [0017] FIGURE 5 illustrates exemplary message exchange in accordance with one or more embodiments of the invention.

[0018] FIGURE 6 illustrates exemplary message exchange in accordance with one or more embodiments of the invention.

[0019] FIGURE 7 illustrates exemplary message exchange in accordance with one or more embodiments of the invention.

[0020] FIGURE 8 illustrates exemplary message exchange in accordance with one or more embodiments of the invention.

[0021] FIGURE 9 describes an exemplary algorithm for performing a method in accordance with one or more embodiments of the invention.

[0022] FIGURE 10 illustrates determination of one or more focal points in accordance with one or more embodiments of the invention.

[0023] FIGURE 11 illustrates an exemplary radiation pattern in accordance with one or more embodiments of the invention.

[0024] FIGURE 12 illustrates an exemplary reference radiation pattern and determination of one or more focal points for a single WSD equipped with a directive antenna, in accordance with one or more embodiments of the invention.

[0025] FIGURE 13 presents an example of determining one or more focal points for a plurality of WSDs equipped with omnidirectional antennas, in accordance with one or more embodiments of the invention.

[0026] FIGURE 14a-g present examples of determining one or more focal points when at least one of a plurality of WSDs is equipped with a directional antenna, in accordance with one or more embodiments of the invention.

[0027] FIGURE 15 illustrates an example of determining a reference direction line, in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION OF THE FIGURES

[0028] The continuous growth of wireless communication systems and their demand for additional bandwidth has motivated the redefinition of spectrum usage in a more efficient way. In many cases, a spectrum may be allocated in a fixed manner, i.e., it may be allocated according to a service or technology. This may lead to a waste of this scarce resource since some amounts of spectrum are not used all the time in every location. At the moment, regulatory bodies around the world are discussing another model of spectrum allocation, which may be adapted to the resource demand in a given location and at certain time period. These discussions concern, at first, the secondary use of the vacant spectrum licensed to digital terrestrial broadcast systems, i.e., TV white spaces. However, the communication on the secondary channels should not, while providing satisfactory communication link for the secondary user, cause any harmful interference to the users of the primary service.

[0029] In order to avoid interference with primary, or incumbent, services, secondary systems may be able to identify vacant channels. In this context, a vacant channel may be understood to be a range of radio frequencies (RF) at a given time period, in which a secondary communication would not cause harmful interference to a primary system. Hence, a vacant channel may also include transmissions from the primary system. Identifying a vacant channel may comprise considering the structure and coverage of the primary network, and determining allowed operating parameters for the secondary user on a vacant channel based on a variety of variables. The allowed operating parameters may include for example a maximum permitted power level, which may be a measured as the equivalent isotropic radiated power (EIRP).

[0030] Defining a maximum permitted power level for a white space device may be resolved by at least two distinct approaches: in the United States the Federal

Communications Commission (FCC) has adopted a fixed approach, in which the maximum permitted power level is defined by the position of the white space device regarding a primary transmitter (inside or outside the coverage area), and by the white space device type (fixed or portable). In Europe, the SE43 group of ECC CEPT (Electronics

Communications Committee - The European Conference of Postal and

Telecommunications Administrations), has been working on a flexible approach, in which the maximum white space device power is defined considering the quality of the digital terrestrial television (DTT) service coverage in a given location, and the maximum permitted degradation of the location probability for a receiver placed at a reference geometry. In example embodiments of the invention, location probability may refer to a measure of quality of the DTT service coverage. More specifically, it may be equal to the probability with which a receiver operates appropriately in a given area. For example, a location probability of 95 % in a given area may mean that the DTT signal level is such that a DTT receiver operates satisfactorily in 95% of that area. A reference geometry may be defined because there may not be information about the actual coupling loss between the WSD transmitter and the DTT receivers, for example because actual locations of the

DTT receivers may not be known. Therefore, some reference scenarios, i.e. reference geometries, may be chosen for the calculations of maximum permitted power levels for the WSDs. These WSD EIRP limits protect the DTT receivers from harmful interference, even if they are calculated considering the worst case configuration.

[0031] In order to avoid harmful interference, e.g., to a primary DTT service reception, maximum EIRP levels for white space devices may be defined. WSD EIRP limits may be defined according to criteria of protection to the primary service, which may take into account limitations of DTT receivers in dealing with interference, as well as location specific aspects like the signal quality of the primary service. Efficient usage of TV white spaces may depend on the flexibility of WSD EIRP limits according to specific locations.

[0032] In spite of the extensive work about spectrum sensing techniques, the secondary user communication in TV white spaces may rely on the channel assignment based on the information provided by a geo-location database, at least on the first stage of its implementation. In order to protect transmissions in the primary system, the geo- location database may guarantee that interference levels caused by secondary users are below regulatory limits. This invention is related to an algorithm to calculate a maximum permitted WSD EIRP considering the aggregate interference caused to a primary service.

[0033] FIGURE 1 presents an exemplary structure of a TV white space network comprising a primary network 100 and a secondary network 120 according to an embodiment of the invention. The primary network 100 may be for example an incumbent DTT network as described earlier in this specification. The primary network 100 may comprise one or more primary base stations (PBS) or primary transmitters 101, and one or more primary users (PU) or primary receivers 102. The secondary network 120 may comprise one or more secondary base stations (SBS) 121, for example private or public network access points. The secondary base station 121 may provide communication services for one or more secondary users or white space devices 122, 123. The white space devices 122 and 123 may be for example personal/portable devices or home/office appliances as described earlier in this specification. The secondary base station 121 may also be a similar white space device as devices 122 and 123, or, one of the white space devices 122, 123 may act as a base station serving other white space devices. Any of the devices 121, 122, and 123 may be able to communicate with each other and/or one or more geo-location databases 110.

[0034] Database 110 may provide cognitive radio controlling services for the secondary network 120. Database 110 may be also comprised in the secondary network 120 inside one or more of the network devices, or as an individual entity. Database 110 may comprise memory for storing information, e.g., spectrum availability information 111 for different geographical locations, primary network information 112, and WSD information 113. The primary network information 112 may further comprise field strength information, e.g., in form of estimated or measured median field strength of the primary transmitter 101 for a plurality of locations within the operating area of the database 110. The primary network information 111 may also include maximum permitted degradation of the location probability for the plurality of locations within the operating area and reference values of protection ratio and overloading thresholds. Protection ratio may refer to a minimum value of the signal-to-interference ratio, at which the DTT receiver may operate appropriately. An overloading threshold may refer to an interference level, above which a DTT receiver begins to lose the ability to discriminate interfering signals at adjacent frequencies from the primary signal. The primary network information 112 may be organized as a map, wherein a plurality of pixels may represent different geographical areas or locations. The spectrum availability information 111, primary network information 112, WSD information 113, as well as any other information may be pre-configured in the memory of the database 110, loaded through a network interface, or received from a wireless radio interface and stored in the memory of the database.

[0035] In an example embodiment, the WSD information 113 may include WSD identity information (WSD ID), information about the locations of WSDs, location accuracy for the WSDs, or further WSD specific information. As further examples, a WSD type parameter may identify whether a WSD is portable or fixed, and whether emission characteristics of the WSD comprise information about at least one of emission mask, antenna type, antenna radiation pattern, or adjacent channel leakage ratio. WSD

information 113 may also include site information; e.g., antenna height and azimuth angle. Each WSD 122, 123 may send this information directly to the database 110, or, as an alternative embodiment the information may be sent to the secondary base station 121, which may then send the received WSD information 113 to the database 110.

[0036] According to an example embodiment of the invention, there may be a plurality of databases 110. The databases 110 may be able to communicate, share, and combine information with each other. The databases may for example be equipped with network interfaces, through which they may be able to send and receive messages. The plurality of databases 110 may also be comprised in a single device. [0037] The secondary network 120 may produce interference 115 to the primary network 100. This aggregate interference 115 may comprise transmissions from the secondary base station 121 and/or the WSDs 122, 123. The invention assures that the caused aggregate interference 115 to the primary system 100 is maintained below regulatory limits, while maximizing the transmission capacity for the devices in the secondary network 120.

[0038] FIGURE 2 presents an exemplary structure of the database 110. Database 110 may include at least one processor 221 in connection with at least one memory 223 or other computer readable media. Memory 223 may comprise any type of information storing medium including for example random access memory (RAM), read-only memory (ROM), programmable readable memory (PROM), erasable programmable memory (EPROM) and the like, and it may contain software 224 in form of computer executable instructions. The software may be organized for example as functional blocks, modules, or objects. Processor 221, memory 223, or software 224 may be seen, for example, as means for processing data, means for performing operations, means for calculating, means for associating information, and means for identifying relations between different data.

Database 110 may be a geo-location database itself or it may be comprised in a geo- location database or a cognitive radio controller. Database 110 may further comprise one or more wireless radio interfaces 225, for example one or more telecom radios, or a wireless local area network (WLAN) radio through which it may send and receive messages to/from the SBS 121 or the WSDs 122, 123. Database 110 may also comprise a wired network interface, through which the database 110 may access a variety of networks, e.g., the Internet or a private network such as an operator network. Database 110 may communicate with other databases via the wired or wireless interface 225, 226. The wireless radio interface 225 and wired radio interface 226 may be seen as means for communicating, means for receiving, or means for transmitting data, information, or messages. Database 110 may further comprise a user interface 227 for enabling user made configurations, and a power supply 228.

[0039] FIGURE 3 presents an exemplary apparatus 300 where one or more embodiments presented herein may be implemented. Apparatus 300 may for example be or be comprised in the white space device 122, or alternatively, it may be or be comprised in the secondary base station 121. Apparatus 300 may comprise at least one processor 302 in connection with at least one memory 303 or other computer readable media. Memory 304 may comprise any type of information storing medium including random access memory (RAM), read-only memory (ROM), programmable readable memory (PROM), erasable programmable memory (EPROM) and the like, and it may contain software 304 in form of computer executable instructions. Software 304 may be organized for example as functional blocks, modules, or objects. Processor 302, memory 303, or software 304 may be seen for example as means for processing data, means for performing operations, means for calculating, means for associating information, and means for identifying relations between different data.

[0040] Apparatus 300 may comprise one or more radio interfaces, for example one or more telecom radios 305 such as GSM (Global System for Mobile communications), WCDMA (Wideband Code Division Multiple Access) or TD-SDMA (Time Division

Synchronous Code Division Multiple Access) transceivers or the like; broadcast radios 306 such as DVB-T2 Lite, CMMB (Chinese Mobile Multimedia Broadcasting), or ATSC-M/H (ATSC Mobile/Handheld) receivers; or short-range radios 307 such as Bluetooth or a wireless local area network (WLAN) radio. Apparatus 300 may use these interfaces for communicating with white space devices 122, 123, secondary base stations 121, or databases 110. The wireless radio interfaces 305, 306, or 307 may be generally seen as means for communicating, means for receiving, or means for transmitting data, information, or messages. Apparatus 300 may further comprise a user interface 308, display 301, and audio input/output 308 for communicating with the user. The apparatus may also comprise a battery 309 or other source for delivering power for various operations performed in the device.

[0041] Apparatus 300 may use its resources, in particular the processor 302 and the memory 303, for various purposes. For example, the device may tune to certain radio channels. A radio channel may comprise a range of frequencies on the electromagnetic spectrum. Examples of the radio channels are the broadcast channels on VHF (very high frequency) and UHF (ultra high frequency) bands, cellular radio channels, and unlicensed radio channels in the ISM (industrial, scientific, and medical) band.

[0042] In accordance with an embodiment of the invention, FIGURE 4 presents an algorithm, which may be performed in a geo-location database such as database 110, e.g., to determine allowed operating parameters for secondary users such as WSDs 122, 123 or the secondary base station 121.

[0043] In Step 401, the database 110 may receive channel request, or in general a resource request, from one or more WSDs 122, 123 or SBS 121. The database may receive the resource request for example through one of its wireless radio interfaces 225 and the resource request may comprise at least part of WSD specific information 113. In particular, the resource request, or alternatively any other message received, may include information about the location of the WSD. The location information of the WSD may for example comprise coordinates and it may also comprise location accuracy information. A plurality of techniques may be used to determine a location of a secondary user, such as: GPS (Global Positioning System), WLAN ID, or a cell ID, etc. For example, a WSD may receive positioning signals from a plurality of satellites to determine its location, or the WSD may use cell ID location tracking of a cellular network. Each technique may have its own accuracy level that may be also informed to the geo-location database. The location accuracy information may be expressed, for instance, in terms of an uncertainty radius around the informed location. The database may consider the worst possible locations in its calculations by including a safety margin against location uncertainty.

[0044] The resource request may also include information about antenna height and transmitting or receiving azimuth of the WSD 122, 123 or the SBS 121. If the WSD or SBS is able to determine its antenna height and transmitting azimuth, it may communicate at least one of the two parameters to the geo-location database. The antenna height and/or azimuth angle may be configured at the equipment during installation. The transmission azimuth may be also determined by other means, such as coordination with other secondary users equipped with steerable antennas. If the WSD or SBS is not aware of its height and azimuth angle, the database may assume a default case. Such a default case may be for example configured to be an omnidirectional antenna placed at the same height as reference primary users. The WSD information received along with the resource request may be in the database associated with a particular WSD ID and the database may determine the channel availability based on the information stored in its memory and the received information.

[0045] In Step 402, the database 110 may associate the received location to a pixel of a map, which may be stored in its memory. The pixel or the location itself may be further associated with a maximum permitted interference 7_max, or, the pixel may already have an association with a maximum permitted interference 7_max. The maximum

interference 7_max may indicate a maximum permitted interference to a primary system at the associated location or in the area of the associated pixel. Interference level 7_max to the primary system may be described as the interference caused to a primary receiver, e.g. a DTT receiver, at certain location. [0046] In Step 403, the database may determine whether there are available, or vacant, channels. If it is determined in Step 403 that there are available channels, the algorithm may, in response to the above determination, proceed to Step 404. If it is determined that there are no channels available, the algorithm may proceed to Step 406.

[0047] In Step 404, the database may, based on the type of the WSD or SBS and the received WSD information, e.g., antenna height, radiation pattern, or transmission azimuth angle, select a reference geometry for the link between the interfering WSD and the interfered primary receiver, e.g., a DTT receiver. The reference geometry may be an agreed spatial configuration of a transmitting WSD and the reference primary receiver, for which protection requirements may be met. The reference geometry may be for example defined by regulation or standardization bodies. The reference geometry may be used to calculate the maximum permitted WSD EIRP.

[0048] In Step 405, the database may calculate appropriate power values for at least one of the WSDs. The database may for example calculate a maximum permitted power level for a white space device in response to a resource request from the white space device. The database may also calculate new permitted power levels, or other parameters, in a joint manner for a plurality of white space devices. In one embodiment, the calculations may be performed when the number of WSDs in the operating area of the database changes or any of the white space devices request changes to their current communication parameters.

[0049] Step 405 may comprise estimation of one or more focal points. A focal point may be defined as a location where a maximum permitted interference is calculated. A reference geometry may indicate the positions where the interference to a DTT receiver is considered. A focal point may be a special point in the reference geometry, where the aggregate interference may reach the highest levels. Step 405 may also comprise calculation of the maximum permitted power levels, P,, for a plurality of white space devices (WSDi ... WSD,). The calculations may take into account regulatory limits, current usage of the spectrum by the secondary users, as well as any other information stored in the database, for example, the spectrum availability information 111, the primary network information 112, or the WSD information 113. The complexity of the actions described above may depend on the type of the WSD antenna, its capability of determining the transmitting azimuth, and/or the quantity of WSDs transmitting in the neighborhood. At least the following scenarios may be possible: 1) single WSD equipped with an

omnidirectional antenna, 2) single WSD equipped with directional antenna, 3) multiple WSDs equipped with omnidirectional antennas, or 4) at least one WSD equipped with directional antenna. FIGURE 9 further illustrates an exemplary implementation of Step 405, in accordance with an embodiment of the invention.

[0050] In Step 406, the database may send a response to a WSD that has requested a channel. In addition, the database may send a message also to other WSDs in its operating area. This may be beneficial if, for example, the performed calculations suggest changing the allowed operating parameters of the other WSDs.

[0051] FIGURE 5 illustrates exemplary communication between a database 110 and a WSD 500 according to at least the embodiment illustrated in FIGURE 4. Device 500 may be a white space device such as WSDs 122 or 123 in FIGURE 1 or the secondary base station 121 in FIGURE 1. In Step 401, the WSD may send a channel request or a resource request 501 to database 110. The resource request may comprise necessary information for the database to respond to the request, for example at least one of location information, location accuracy, WSD information, or site information. The resource request may also include information of the needs or desired operating parameters of the WSD, for example a data rate, bandwidth, or a frequency range. Before Step 406, the database may construct a response message 502, which may inform the WSD about available channels, maximum permitted power levels or other operating parameters. The response may be organized as a list of channels, list of available channels, or list of channels associated with a maximum permitted power level. The response message 503 may also inform the WSD that the database has denied access to the requested channel, or that there are no channels available, or that there are no channels available with the desired operating parameters requested by the WSD.

[0052] Embodiments of the invention may implement various scenarios for sending the response message 502. In a first scenario there may be at first only one device operating inside a pixel, and the database may have assigned a maximum permitted power level that leads to 7_max at a selected focal point. In this embodiment, the database may change the maximum permitted power assigned to the first WSD in order to permit operation of other WSDs in that location or elsewhere in the operating area of the database. Furthermore, there may be at least two different embodiments according to the first scenario.

[0053] FIGURE 6 illustrates exemplary communication between a plurality of

WSDs or SBSs 600 and a database 110 according to an embodiment of the invention. In this embodiment, the database 110 may update the maximum permitted power level, or maximum EIRP, for the first WSD by recalculating the maximum permitted power level, and/or any other operating parameters, at each expiration time of information. The communication may proceed as follows: A first WSD may request a channel by sending a resource request message 601 to the database. The database may respond by response message 602, which may include an expiration time for the information embedded in the response message, or any other timing information. Alternatively, or additionally, the response message 602 may include an amount of time for which the information is valid. Based on the received response message, the first WSD may determine a validity period 603 for the information. After sending the operating parameters 602 to the first WSD, another WSD (n^th WSD) may send a channel request 604 to the database. This request for new resources may cause changes also to the currently allocated resources, in particular, the maximum permitted power level of the first WSD. Meanwhile, the first WSD may send one or more resource requests 605 to maintain the current channel. In an example embodiment, the database may wait for the expiration time of the information provided to the first WSD until calculating updated operating parameters considering the plurality of WSDs 600 in its operating area (7.) and sending the new operating parameters to the first WSD and the n^th WSD in messages 606. Alternatively, the database 1 10 may recalculate the updated operating parameters as soon as it has the new WSD information available, but the database may still wait for the expiration time until sending the updated parameters to the plurality of WSDs 600. In particular, the update messages 606 may comprise the maximum permitted power levels for each of the plurality of WSDs 600. The updated operating parameters may have been calculated considering the aggregate interference of the WSDs and SBSs 600 currently active in the operating area of database 1 10.

[0054] Another example embodiment is illustrated in FIGURE 7, where the database 1 10 may broadcast new operating parameters, e.g., maximum permitted power levels, to white space devices located in that region every time a new WSD requests a resource. Therefore, a WSD may receive a new maximum permitted power level or a new resource allocation even during the validity period of the previously received parameters. The procedure may begin similarly to FIGURE 6: a first WSD of the plurality of WSDs 700 may request a channel by sending a resource request message 701 to the database. The database may respond by a response message 702, which may include an expiration time for the information embedded in the response message, or any other timing information.

Alternatively, or additionally, the response message 702 may include an amount of time for which the information is valid. The response message 702 may thus indicate a validity period 703 for the information. After sending the operating parameters 702 to the first WSD, another WSD (n^th WSD) may send a channel request 705 to the database. This request for new resources may cause changes also to the currently allocated resources, for example, the maximum permitted power level for the first WSD. The database may calculate the optimum power allocation considering the plurality of WSDs 700 in its operating area. The database may send the recalculated operating parameters during the validity period 703 of the first WSD in messages 705. In messages 705, the database 1 10 may maintain the channel allocation for the first WSD, but update the permitted maximum power level (maximum EIRP) of the first WSD. Alternatively, the database may change the whole resource allocation in the secondary network, and communicate such changes in messages 705 to the plurality of WSDs 700.

[0055] An example embodiment of the invention according to a second messaging scenario is presented in FIGURE 8. In this embodiment, the database 1 10 may not assign the maximum permissible EIRP to any of the plurality of WSDs 800 in the network.

Therefore, there may always be resources that may be assigned to a new WSD entering the network, at least by a temporary fashion. The maximum permitted power level may be recalculated in accordance with the time expiration of the information. The procedure may begin similarly to FIGURE 6: a first WSD of the plurality of WSDs 800 may request a channel by sending a resource request message 801 to the database 1 10. Database 1 10 may respond by a response message 802, which may include an expiration time for the information embedded in the response message, or any other timing information.

Alternatively, or additionally, the response message 802 may include an amount of time for which the information is valid. The response message 802 may thus indicate a validity period 703 for the information.

[0056] After sending the operating parameters 802 to the first WSD, another WSD

(n^th WSD) may send a channel request 804 to the database. The database may calculate temporary operating parameters for the n ^h WSD taking into account the current operating parameters of the first WSD or all active WSDs in the operating area, in particular, the current maximum permitted power levels for each of the plurality of WSDs. The database 1 10 may send these temporary operating parameters to the n ^h WSD with response message

805 and the n^th WSD may begin to operate with the temporary parameters. Hence, the first

WSD that was already operating may not need to adapt its transmission to any new constraints. After calculating and sending the temporary operating parameters to the n ^h

WSD, the database 1 10 may calculate more optimal resource allocation for the secondary network taking into account the updated WSD information. At the end of the validity period 803, the first WSD may request to maintain the channel by message 806. After the validity period 803 has expired, the database 110 may send the updated operating

parameters for each of the WSDs 800 by update messages 807. In particular, the update messages 807 may comprise the maximum permitted power levels for the plurality of

WSDs 800. The updated operating parameters may have been calculated considering the aggregate interference of the plurality of WSDs and SBSs 800 currently active in the operating area of database 110.

[0057] FIGURE 9 describes an exemplary algorithm in accordance with an

embodiment of the invention. The algorithm may be performed between Steps 404 and 406 as described in FIGURE 4, or, at any other phase. The algorithm may be implemented for example in geo-location database 110.

[0058] In Step 900, database 110 may determine whether a WSD is the only WSD in the operating area of the database. The database may for example process the WSD information 113 stored in its memory to make this determination as well as any of the following determinations. If it is determined in Step 900 that a WSD is the only WSD in the area, the algorithm may proceed to Step 901 to determine whether the WSD has an omnidirectional antenna. If it is determined in Step 900 that the WSD is not the only WSD in the area, the algorithm may proceed to Step 902 to determine whether there is at least one WSD with a directional antenna.

[0059] If it is determined in Step 901 that the WSD has an omnidirectional antenna, the algorithm may proceed to Step 911. In accordance with an embodiment, this branch of the algorithm may correspond to a first scenario where a single WSD may be equipped with an omnidirectional antenna, an example of which is illustrated in FIGURE 10. In this embodiment, one or more focal points (FP) may be determined and/or one maximum

permitted transmit power level may be calculated for a single WSD with an

omnidirectional antenna. In case of a WSD 1002 equipped with an omnidirectional

antenna, the focal point 1001 (FP_1:L), may be any point located at a distance 1003 (d_ref), determined by a reference geometry. A maximum permitted power level for WSD 1002 may be calculated as

Pi = Im_ax / G_n (Eq. 1) where G_l may represent an estimate of a channel gain between WSD transmitter 1002 and focal point 1001. Factor Gn may be for example obtained by propagation models such as Okumura-Hata, Extended Hata for short-range devices, free space path loss and the like. Factor Gn may also comprise statistical components commonly used to model the time- varying nature of the channel. For short distances, e.g. in case of FPs, these models may provide similar values. Note that the interference caused by WSD 1001 may not exceed the level I_max at focal point 1001 (FP_1:L), i.e., the constraint of l_WSD < I_max may be met at focal point 1001. In relation to FIGURE 9, calculating the maximum permitted power in Step 911 may be done using Eq. 1.

[0060] In response to determining in Step 901 that a single WSD does not have an omnidirectional antenna, the exemplary algorithm of FIGURE 9 may proceed to Step 921. This branch of the algorithm may correspond to a second scenario where a single WSD may be equipped with a directional antenna, in accordance with an embodiment of the invention. In this embodiment, one or more focal points may be determined and/or the maximum permitted WSD power level may be calculated for a single WSD.

[0061] FIGURE 11 illustrates an example of a radiation pattern according to an embodiment of the invention. In FIGURE 11 , antenna discrimination (loss) is shown as a function of the angle. In this example, the antenna discrimination is not constant as a function of angle and the loss ranges from 0 dB to 1 dB between 0° and 20° from the main lobe 1101, and from 1 dB to 17 dB between 20° and 60° from the main lobe 1101. In accordance with an embodiment, simplified reference radiation patterns may be used for determining one or more focal points or for calculating maximum permitted power levels at the geo-location database. The region of the best response may be for example identified by the angles around the main lobe, corresponding to the minimum discrimination and some dBs of additional discrimination. Moreover, a certain level of discrimination can be considered for the angles corresponding to side lobes 1102 and the back lobe 1103. In case of FIGURE 11, for instance, a reference radiation pattern could define the region of best response as the one from 0° to 30° from the main lobe, where the discrimination ranges from 0 dB to 3 dB, and a discrimination of 13 dB for angles beyond 55° from the main lobe.

[0062] According to an embodiment of the invention, a reference radiation pattern may be described by one or more reference directions. FIGURE 11 illustrates such exemplary reference directions 1104 and 1105. Reference directions 1104 and 1105 which may limit the area of reference radiation pattern, and they may be assigned around the main lobe 1101. In alternative embodiments, the reference directions may be assigned considering also back slobe 1103 or side slobes 1102. An example of a reference radiation pattern 1204 and reference directions 1205 are also shown in FIGURE 12. In embodiments of the invention the reference radiation pattern may denote the sector limited by reference direction lines 1205. In other words, the radiation pattern may not be limited to any

distance from the WSD.

[0063] FIGURE 12 further illustrates an example of determining a focal point 1201

(FPn) according to an embodiment of a WSD 1202 equipped with a directional antenna, the focal point 1201, may be any point located at a distance 1203 (d_ref) in the region of best response of the antenna. Considering Α^φ-^), an antenna discrimination as a function of angle, a maximum permitted power level for transmission may be calculated as Pi = I_max / (G_n * minfAfa }) (Eq. 2) where Gn may represent an estimate of a channel gain between WSD transmitter 1202 and the focal point 1201 (FPn), obtained with the use of propagation models as described earlier, and min{A(0₁)] may be a minimum of the antenna discrimination. The interference caused by WSD 1201 may advantageously not exceed I_max at focal point 1001 (FP_1:L), i.e., the constraint of I_WSD≤ I_max may be met at the focal point 1201 (FP_1:L). In relation to

FIGURE 9, calculating the maximum permitted power in Step 921 may be done by Eq. 2.

[0064] Returning to FIGURE 9, in response to determining in Step 902 that there are no WSDs with directional antennas, the example algorithm may proceed to Step 931.

This branch of the algorithm may correspond to a third scenario, where one or more focal points are determined and/or one or more maximum permitted power levels are calculated for a plurality of WSDs equipped with omnidirectional antennas, according to an example embodiment of the invention.

[0065] In an embodiment of N WSDs transmitting in the operating area of a geo- location database, the focal points may be determined considering the interference

generated by all of them. In case of omnidirectional antennas, the database may consider the locations provided by each WSD and the distance d_ref, which may be a reference

geometry parameter determined by regulatory authorities. Each WSD in a group of N

WSDs located in a given area may have focal points FPy related to the N— 1 other WSDs. FIGURE 13 presents an example of determining focal points for four WSDs: WSDi ,

WSD₂, WSD₃, and WSD₄. Each WSD, may be associated with coordinates (xi,yi) and may have three focal points

For example, WSDi may be associated with coordinates (xi, yi) and have focal points FP₁₂, FPn and FPi₄. The coordinates may be informed by each WSD to the database. A distance between each pair of WSDs, dy = ά , may be estimated by the geo-location database. An angle between two WSDs, α¾·, compared to a reference angle, a₀, may be written as ay = arctan (^— ¹ ) (Eq. 3)

[0066] For each WSD i, j = 1, ... , N— 1 angles may be considered. The focal points, FPj_j , may be determined from and d_ref as follows:

FPi_j = (xi. yj + (d_refcos( otj_j ) , d_refsin( ο¾ )) (Eq. 4)

[0067] After determining the focal points, FPj_j, the database may calculate the maximum permitted power level, P, , for all WSDs. The calculations may consider the aggregate interference at the focal points, which may be estimated through the estimates of channel gain between each j^th WSD transmitter and the focal point at the reference

geometry of the i^th WSD. It is possible to maximize the permitted power values while guaranteeing that the aggregate interference at all focal points stays smaller than the

maximum permitted interference to a primary user at each focal point. The problem may be formulated as the following linear program: max c^TP,

subject to GP < I_max (Eq. 5)

P > 0 where c = [1, \, ... , 1]^T is a [N x 1] vector, P = [Ρ_1; P₂, ... , P_N]^T is the [N x 1] power vector to be optimized, I_max = [l_max,i I_max,2 ■■■ ΙΠΙΗΧ,Ν] is [N x 1] vector that contains the maximum permitted interference at the location provided by the WSD and G is the

[N(N— 1) x N] matrix that contains all channel gain estimates:

[0068] The power vector P in the problem above may be efficiently obtained by linear programming algorithms, e.g., a simplex algorithm or interior point methods. The system of linear inequalities in Eq. 5 may define a convex polytope as a region of feasible solutions. The simplex algorithm may start from a vertex of the feasible region and move along the adjacent vertices of the polytope until reaching the vertex of the optimum

solution. In contrast, interior point methods may search for the optimum solution by cutting a clever path in the interior of the feasible region. In accordance with FIGURE 9, one or more power values may be calculated in Step 932 by solving Eq. 5.

[0069] As discussed above, Step 902 in the example algorithm of FIGURE 9 may determine whether at least one of a plurality of WSDs located in an operating area of a geo-location database has a directional antenna. If it is determined in Step 902, that at least one WSD has a directional antenna, the algorithm may proceed to Step 941. A directional antenna may refer to an antenna that, for example, has a radiation pattern similar to FIGURE 11. This branch of the example algorithm may represent a fourth scenario where one or more focal points are determined and/or one or more maximum permitted power levels are calculated for at least one of a plurality of WSDs, in accordance with an embodiment of an invention.

[0070] In this example embodiment, the geo-location database may take into account the antenna radiation pattern and/or transmission azimuth angle for each WSD. A number of different example cases are possible, depending on the azimuth angle of each transmitter. These example embodiments are illustrated in FIGURE 14a-g. The method, apparatus, or computer program disclosed herein may be advantageously used to determine focal points and calculate the maximum permitted power levels for each embodiment. The algorithm may identify the focal points FP_£ at WSD; due to the presence of WSD_j, where k represents the type of the focal point.

[0071] FIGURE 14a illustrates an embodiment, where antennas of two WSDs,

WSDi and WSD₂ are aligned. The focal points may be discovered similarly to the case in which all WSDs are equipped with omnidirectional antennas and there is only one focal point for each neighbor WSD. This focal point may be named FP^ , where indices i,j represent the WSDs involved in the determination.

[0072] FIGURE 14b illustrates an example embodiment, where antennas of two

WSDs WSDi and WSD₂ are not aligned, but the areas covered by the reference radiation pattern 1404 of WSD₂ and reference radiation pattern 1045 of WSDi are overlapping inside the reference geometry of WSDi in area 1406. The reference radiation pattern 1404 and transmission azimuth angle θ₂ of WSD₂ may be modeled by defining one or more reference directions 1401 and 1403, which may limit the reference radiation pattern 1404 of WSD₂. The reference geometry 1402 of WSDi may be modeled as a circle around the location of WSDi. The radius of the circle may be equal or be derived from reference geometry parameters provided by a regulator. According to an embodiment of the invention, a focal point FP^- may be determined at an intersection point between a reference direction, e.g. reference direction 1401 of WSD₂, and reference geometry, e.g. , reference geometry 1402 of WSDi. The database may therefore consider the focal point at the reference geometry of WSD 1, FP^, and the intersection between the limiting direction of maximum gain of WSD₂ and the reference geometry of WSDi, FP^ .

[0073] FIGURE 14c illustrates an example embodiment, where antennas of two WSDs WSDi and WSD₂ are not aligned, the radiation patterns of the WSDs do not overlap inside the reference geometry of WSDi_, but there is still an intersection between reference direction 1401 of WSD₂ and reference geometry 1402 of WSDi. In this embodiment, the database may determine the focal points to be FP^, FP^ or FP^, where FP^ may represent a limiting point at the reference geometry of WSDi at which one observes the best response of the antenna of WSDi . Considering FP^ is relevant because it receives the same interference from WSDi as FP^, but it is closer to the region of maximum gain of WSD₂ and therefore it may be subject to high aggregate interference.

[0074] FIGURE 14d illustrates an example embodiment, where antennas of two

WSDs WSDi and WSD₂ are not aligned, there is no overlapping between the reference radiation patterns of the two WSDs inside the reference geometry of WSDi, and reference directions of WSD₂ do not intersect with reference geometry of WSDi. In this

embodiment, the database may determine the focal points to be FP^ or FP^ .

[0075] FIGURE 14e illustrates an example embodiment, where the reference geometry of WSDi is entirely inside the reference radiation pattern of WSD₂. In this embodiment, the database may determine the focal points to be FP^ or FP^ .

[0076] FIGURE 14f illustrates another example embodiment, where the reference radiation pattern of WSD₂ overlaps with the reference radiation pattern of WSDi inside the reference geometry of WSDi. There is also at least one intersection point between a reference direction 1401 of WSD₂ and the reference geometry 1402 of WSDi. In this embodiment, the database may determine the focal points to be FP^, FP^ or FP^ .

[0077] FIGURE 14g illustrates another example embodiment, where the radiation patterns of the WSDs do not overlap inside the reference geometry 1402 of WSDi , but there is still at least one intersection point (FP^, FP^) between reference direction 1401 of WSD₂ and reference geometry 1402 of WSDi. In this embodiment, the database may determine the focal points to be FP^, FP^, FP^ or FP^ . [0078] In accordance with example embodiments of the invention, the different types of focal points FP_£ identified in accordance with FIGURE 14 may be determined as follows:

[0079] The first type of focal points, FPl , may be placed over a line that connects WSD; and WSD_j . This may be also the focal point that may be considered in case of

omnidirectional antennas (Eq. 4). FP^ may be determined to be at the intersection of a connecting line between WSD, and WSD, and the reference geometry of WSD,.

[0080] The second and third type of focal points FP^and FP^- may be determined as the points of intersection between a reference direction of WSD,- and the reference

geometry of WSDj. These points may not exist in all cases, as illustrated in embodiments of FIGURE 14de. A circle that delimits the reference geometry 1402 of WSD^ may be described by

(x - Xi) ² + (y - _yi) = d_r ² ₍ (Eq. 7) wherein (x_it yi) may represent the coordinates of WSD_{i ?} and d_ref may be the distance of the reference geometry 1402, e.g., the radius of the circle. It is to be understood that any other geometrical shapes may be used instead of a circle without departing from the spirit of the invention.

[0081] WSD_[ and WSD,- may be capable of informing their transmission azimuth angles, Q_t and θ,· , to the database. The WSDs may also inform their radiation pattern, or one or more reference directions 1401 , 1403 to the database. Alternatively, this

information may be already available at the database, e.g., associated with the WSD

information 1 13.

[0082] FIGURE 15 illustrates an example of determining one or more reference directions 1501 , in accordance with an embodiment of the invention. A reference direction may be represented by a line 1501. Angle Θ,· may be a transmission azimuth angle of

WSDy with respect to a reference angle 1503. Angle β · may be defined as a half angular aperture with best response of the antenna, and it may represent the angle of reference direction 1501 with respect to the transmission azimuth angle Θ,· of WSD_y. The line of the reference direction 1501 , which may delimit the region of maximum gain of WSDy may be described as

[0083] Focal points FP^ and FP^- may be determined at the intersection points between the line given by Eq. 8 and the circle defined by Eq. 7. Substituting the

coordinates given by Equation 8 into Equation 7:

2 2

(_Xj + d * cos (θ; + β.) - _Xi) + (y. + d * sin (θ_{ + β.) - y.) = d_r ² _ef (Eq. 9) which gives d² + [2 (Ax cos( 6_j + β.) + Aysin G_j + β.)] d + ((Δχ)² + (Ay)² - d² _ef) = 0 (Eq. 10) where Δχ = X — x_£ and Ay = y,-— y_£ . Eq. 9 is a quadratic equation in d and it may have zero, one or two real solutions. Considering that d' is a real solution of Eq. 9, focal points may be calculated as:

FP_ij = (x_j,y.) + d^' * (cos ( e_j + β.) _{> 5}ίη (θ_{ + β.)) (Eq. 1 1)

[0084] The fourth and fifth type of focal points FP_£ and FP^ may delimit a region of maximum gain of WSD,. Calculation of these focal points may depend on the coordinates of WSD; (xj, yi) or the information of the radiation pattern and transmission azimuth angle θ[. Focal points FP_£ and FP^ may be calculated as

FP_; = (xi, y.) + d_ref * (cos( θ_; ± β.) , sin(6_I ± β.)) (Eq. 12)

[0085] Each focal point specified above may correspond to a constraint in the linear programming problem. For each WSD,, and for each focal point FP^ associated with

WSD_[ and WSD_m the following constraint may apply, considering N - 1 neighboring

WSDs and their influence in terms of interference at that focal point:

(WSD, FPL) : P, * (G^ * A (φ^ + - + P_N * (c * A (φ^™) < I_MAX, (Eq. 13) where A ^Φ^'"¹^ ^maY be an antenna attenuation of the 7^th WSD regarding the k^th focal point FP^_j, determined between WSD_£ and WSD_m. As in the embodiment of

omnidirectional antennas, this is a linear programming problem. An exemplary difference may be that the matrix G in the embodiment of directional antennas may contain also influence of antenna attenuation. The embodiment of directional antennas may also include more matrix elements, since each link between two WSDs may have more than one focal point. [0086] In accordance of FIGURE 9, the exemplary algorithm may in Step 941 determine one or more focal points based on Eq. 4, Eq. 11, or Eq. 12 as described above. In Step 942, the algorithm may solve problem described in Eq. 13 to calculate the maximum permitted power levels for a plurality of WSDs.

[0087] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is to maximize resource usage in a cognitive radio network. This objective may be accomplished by advantageously maximizing allocation of maximum permitted power levels, while guaranteeing that an aggregate interference at one or more focal points at reference geometries stays below regulatory limits. Another technical effect of the embodiments of the invention is to maximize resource usage without additional network elements. A further benefit of the invention is the formulation of the optimization problem in a linear programing form, which may be solved by fast and low-complexity algorithms.

[0088] Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and/or hardware may reside on a cognitive radio device such a cognitive radio controller, database, a mobile phone, laptop, handheld computer, portable music device, accessory or the like. In general, such device may include means for computing and means for storing data on a readable memory in order to perform the functionality according to one or more embodiments of the invention.

[0089] In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any non-transitory media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer, with one example of a computer described and depicted in FIGURE 2 or FIGURE 3. A computer-readable medium may comprise a computer-readable storage medium that may be any non-transitory media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0090] If desired, the different functions or algorithm steps discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions or steps may be optional or may be combined. [0091] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

[0092] It is also noted herein that while the above describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS

1. A method, comprising: receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; receiving information about at least one location of at least one second white space device; associating the location of the first white space device with a first maximum interference; associating the at least one location of the at least one second white space device with at least one second maximum interference; identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and informing the maximum permitted power level to the first white space device.

2. The method according to claim 1, wherein identifying the one of more focal points comprises determining at least one intersection point between a line from the location of the first white space device to the location the at least one second white space device and a reference geometry of the second white space device.

3. The method according to claim 2, wherein the reference geometry comprises a circle around the first white space device and the radius of the circle is determined based on a distance to a reference receiver of a primary network.

4. The method according any of previous claims, wherein identifying the one or more focal points further comprises:

determining at least one reference direction line in accordance with a radiation pattern of the first white space device; and

determining at least one intersection point between the at least one reference direction line and a reference geometry of the second white space device.

5. The method according to claim 4, wherein the at least one reference direction line is determined based on antenna discrimination.

6. The method according to any of the previous claims, wherein the maximum permitted power level is jointly calculated for the first white space device and the at least one second white space device; the calculations further comprising:

maximizing a power vector P subject to constraint GP < I_max, wherein G is a propagation factor matrix with elements representing propagation factors from the first white space device and the at least one second white space device to the one or more focal points, and wherein I_max is a vector of maximum interferences at the focal points.

7. The method according to claim 3, wherein the resource request further comprises a location accuracy, and wherein identifying the one or more focal points further comprises adding a location accuracy margin to the distance to the reference receiver.

8. The method according to any of previous claims, wherein the first and second maximum interferences comprise regulatory limits assigned to the locations of the first and the at least one second white space devices.

9. An apparatus, comprising:

at least one processor;

at least one memory containing executable instructions, wherein the executable instructions, when processed by the processor, cause at least to: receive a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; receive information about at least one location of at least one second white space device; associate the location of the first white space device with a first maximum interference; associate the at least one location of the at least one second white space device with at least one second maximum interference; identify one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; calculate a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and inform the maximum permitted power level to the first white space device.

10. The apparatus according to claim 9, wherein the executable instructions further cause to determine at least one intersection point between a line from the location of the first white space device to the location the at least one second white space device and a reference geometry of the second white space device.

11. The apparatus according to claim 9, wherein the reference geometry comprises a circle around the first white space device and the radius of the circle is determined based on a distance to a reference receiver of a primary network.

12. The apparatus according to any of claims 9 to 11, wherein the executable instructions further cause to:

determine at least one reference direction line in accordance with a radiation pattern of the first white space device; and

determine at least one intersection point between the at least one reference direction line and a reference geometry of the second white space device.

13. The apparatus according to claim 9, wherein the at least one reference direction line is determined based on antenna discrimination.

14. The apparatus according to any of claims 9 to 12, wherein the executable instructions further cause to jointly calculate the maximum permitted power level for the first white space device and the at least one second white space device; the calculations further comprising:

15. The apparatus according to claim 11, wherein the resource request further comprises a location accuracy, and wherein identifying the one or more focal points comprises adding a location accuracy margin to the distance to the reference receiver.

16. The apparatus according to claims 9 to 15, wherein the first and second maximum interferences comprise regulatory limits assigned to the locations of the first and the at least one second white space devices.

17. A computer program product comprising a computer-readable medium bearing computer program code embodied therein for use with a computer, the computer program code comprising: code for receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; code for receiving information about at least one location of at least one second white space device; code for associating the location of the first white space device with a first maximum interference; code for associating the at least one location of the at least one second white space device with at least one second maximum interference; code for identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; code for calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and code for informing the maximum permitted power level to the first white space device.

18. An app aratus comprising : means for receiving a resource request from a first white space device, wherein the resource request comprises a location of the first white space device; means for receiving information about at least one location of at least one second white space device; means for associating the location of the first white space device with a first maximum interference; means for associating the at least one location of the at least one second white space device with at least one second maximum interference; means for identifying one or more focal points based on the location of the first white space device and the at least one location of the at least one second white space device; means for calculating a maximum permitted power level for the first white space device at least partly based on determining an aggregate interference level caused by the first white space device and the at least one second white space device at the one or more focal points, wherein a propagation model is applied to estimate the interference level caused by the first white space device and the at least one second white space device at the one or more focal points; and means for informing the maximum permitted power level to the first white space device.